1. Field of the Invention
This invention relates to polar polymers and microemulsions for the chemically enhanced recovery of oil. More particularly, the invention relates to polymer-microemulsion complexes in which the polymer is complexed with the surfactant in the microemulsion and a method of using the complexes for the secondary or tertiary recovery of crude oil.
2. Description of the Prior Art
Substantial amounts of crude oil remain trapped in subterranean formations after primary recovery techniques, which rely on the initial formation pressure for production have been completed. Efforts to recover additional amounts of oil involve secondary and tertiary techniques. A common secondary technique involves water flooding or the injection of an aqueous solution under high pressure to displace oil. Water flooding, however, results in the formation of discontinuous globules of oil which are trapped within the formation by capillary forces. In order to release this trapped or residual oil, it is necessary to contact residual oil with a fluid which will interact with the oil causing it to be displaced. This chemically enhanced recovery technique requires a substantial reduction in the interfacial tension between oil and water to values less than .about.0.1 dyne/cm.
One method of lowering interfacial tension utilizes microemulsions which contain an oil, water or brine and sufficient amounts of one or more surfactants to solubilize the oil in the brine. While microemulsions have been used to displace residual oil, their effectiveness remains limited by several factors. First, typical microemulsions have low viscosities. When a less viscous fluid is used to displace a more viscous fluid, an instability known as fingering is created at the fluid interface which results in decreased productivity due to an unfavorable mobility ratio. Second, the surfactant is frequently adsorbed by the formation, which results in loss of the microemulsion bank or microemulsion destabilization due to loss of surfactant. Alternatively, one or more components of the surfactant system can be selectively adsorbed by the formation. This chromatographic action tends to shift the hydrophilic-lipophilic balance (HLB) in the surfactant system which can lead to a destabilization or shift to a region of higher interfacial tension on the oil-water-surfactant phase diagram. Third, the effectiveness of any given microemulsion system is often limited by a failure to "bank oil" early in the production process, i.e., soon after injection into the oil field.
U.S. Pat. Nos. 3,282,337 and 3,692,113 disclose that polyethylene oxide can be used as a thixotropic agent for increasing the viscosity of injection water, thus improving the mobility ratio. It is known from U.S. Pat. No. 3,915,230 to use a viscosity increasing hydrophilic polymer solution of optimal salinity and hardness as a preflush for displacing formation water to enhance the action of a subsequently injected surfactant. U.S. Pat. No. 3,704,990 also discloses a water soluble polymer preflush.
When surfactant adsorption is or is expected to be a problem, U.S. Pat. No. 4,016,932 teaches the use of various inorganic or organic sacrifical adsorption agents. Hydrophilic polymers may also be added as viscosity increasing additives.
With respect to the effect of polyethylene oxide and related polymers on other aspects of the rheological properties of fluids used in chemically enhanced oil recovery, U.S. Pat. No. 3,946,811 discloses that the deleterious effects of divalent and trivalent cations can be minimized by employing a polyalkene oxide preslug. It was considered likely that the polyvalent cations formed weak bonds with dipolar sites on the polymer molecule. British Pat. No. 1,162,414 teaches that polyethylene oxide and petroleum sulfonates provide a composition with substantially Newtonian characteristics at low shear rates and dilatant characteristics at high shear rates, and U.S. Pat. No. 3,882,939 describes a polyalkylene oxide polymer complexed with a resin such as a lignosulfonate. U.S. Pat. No. 4,124,073 teaches that thiourea functions as a solution viscosity stabilizer in aqueous compositions comprising thiourea, nonionic linear or branched polyalkylene oxides such as polyethylene oxide and anionic surfactants. Example III is directed to a Berea core test.
Finally, B. Cabane (J. Phys. Chem., 81, 1639 (1977)) in a paper entitled, "Structure of Some Polymer-Detergent Aggregates in Water," discussed the interaction of polyethylene oxide and sodium dodecyl sulfate. The author concluded that the polymer/sulfonate aggregate was a mixed micelle in which the local structure was determined by the strong forces between the detergent molecules while the polymer is weakly adsorbed on the surface of the micelle. K. Shirahama (J. Colloid & Polymer Sci., 252, 978 (1974)) has determined that polyethylene oxide complexes with sodium dodecyl sulfate by ion-dipole forces to the ether oxygen. The reported bond strength is in the order of 6 Kcal/mole and the interaction is molecular weight dependent such that no association energy is observed below about a molecular weight of 4,000.
It would be highly desirable to have a microemulsion which would simultaneously act as a viscosifying agent, resist destabilization due to selective adsorption and chromatography, be stable to dilution and salinity changes while at the same time assist in the early "banking" of oil.